1. Field of the Invention
This invention relates to a flexible magnetic disk cassette in which a flexible magnetic disk is rotatably accommodated, and relates to a recording and/or reproducing apparatus for the same.
2. Description of the Prior Art
As a positioning (centering) unit for a magnetic disk having concentric or spiral recording track, there has heretofore been known such a positioning unit as shown in FIG. 1. Referring to FIG. 1 for illustration of a conventional positioning unit of this sort, a disk cassette 1 comprises a cassette cover 4 in the form of a rectangular parallelopiped composed of upper and lower halves 2 and 3, and a flexible sheet-like magnetic disk (magnetic sheet) 5 rotatably accommodated within a space 4a of the cassette cover 4.
The magnetic disk 5, for example, has magnetic surfaces formed on both upper and lower surfaces thereof and also has a disk mounting aperture 6 formed in its central part. On the other hand, in an upper plate 2a and a lower plate 3a of the disk cassette 1 there are formed a pair of upper and lower air inflow apertures 7 and 8 in corresponding relation to the disk mounting aperture 6, and air outflow apertures 9 and 10 in four points of the circumferential part of the space 4a. Furthermore, a pair of upper and lower head insertion apertures 11 and 12 comprised of apertures extending radially of the 4a, respectively, are provided.
The disk cassette 1 is installed (loaded) by a guide member 14 onto a predetermined position of a magnetic recording and reproducing apparatus, and the magnetic disk 5 is fixed to a disk rotating shaft 17 by being chucked between a driving shaft 15 and a pressure member 16 at the portion of the disk mounting aperture 6, whereby the magnetic disk 5 is positioned with respect to the driving shaft 15.
On the other hand, into the head insertion aperture 12 below the disk cassette 1 there is inserted a magnetic head 18, which is brought into contact with the lower magnetic surface of the magnetic disk 5.
In such a cassette loaded state, the disk rotating shaft 17 is rotated by a motor 19 whereby the magnetic disk 5 in the disk cassette 1 is rotated, and in association therewith the magnetic head 18 is moved in a radial direction of the magnetic disk 5 to perform a desired recording or reproducing operation.
However, in case the magnetic disk 5 is a flexible sheet-like disk, the disk mounting aperture 6 in the magnetic disk 5 may partially be torn up and deformed as shown in FIG. 2, or the peripheral edge portion of the aperture 6 may be damaged by the driving shaft 15, when mounting the magnetic disk 5 on the driving shaft 15. Once the aperture 6 is so deformed or damaged, it becomes impossible to effect positioning, namely centering, of the magnetic disk 5 with respect to the driving shaft 15 with high accuracy. As a result, at every installation (loading) of the magnetic disk 5, its position with respect to the driving shaft 15 varies, and this impedes a high accuracy reproduction. The conventional positioning mechanism shown has involved such an inconvenience.
Though not shown, moreover there also is known a positioning unit wherein a reinforcing ring formed of a metal or synthetic resin is attached to the center aperture of a magnetic disk. Also in this case, however, it is very difficult to avoid a positional shift of the magnetic disk when installed, namely an amount of deviation of the center of the magnetic disk with respect to the axis of the driving shaft, within the tolerance of the diameter of the driving shaft and that of the above center aperture.
Consequently, the conventional positioning system just mentioned above has involved an inconvenience such that every time the magnetic disk is installed there occur variations in its installed position, so it is impossible to install the magnetic disk always in the same state with respect to the driving shaft. Also, the positional shift of the recording track on the magnetic disk with respect to the driving shaft is too large. So there occurs a noticeable tracking error which impedes the attainment of a high density recording track.